1. Field of the Invention The present invention relates to an additional-information detection processing apparatus and method, a content playback processing apparatus and method, and a computer program. More specifically, the present invention relates to an additional-information detection processing apparatus and method, a content playback processing apparatus and method, and a computer program in which the distribution of illegal content, such as copy data of content data, can be effectively prevented.
2. Description of the Related Art
With the progress of digital technology, the dissemination of digital recording and playback apparatuses capable of recording and playing back many times without degradation in image or sound quality has increased, and a variety of digital content such as images and music can be delivered and distributed via media such as digital video tape recorders (VTRs), digital versatile discs (DVDs), and compact discs (CDs), or over networks.
In a known method, various types of information, such as content copyright information, content modification information, content configuration information, content processing information, content editing information, content playback processing schemes, and content copy control information, is embedded into the content as additional information corresponding to the content using a digital watermark (WM) technique. Digital watermarks are typically invisible or imperceptible on played back content (image data or audio data), and can be detected or embedded only by executing a specific algorithm or by a specific device. Content is processed in a receiver, a recording and playback apparatus, or the like to detect a digital watermark to control the content according to the digital watermark, thus achieving highly reliable control.
A variety of methods of embedding and detecting digital watermarks in and from data have been proposed. A typical method of embedding and detecting digital watermarks is based on the statistical characteristics of original signal data such as an image. A description is now given of a method of embedding a digital watermark as a basic pattern of random-number data of a PN (pseudo-noise) sequence based on the statistical characteristics of an image signal such as a digital video signal. For simplification, it is assumed herein that frame data of a luminance signal has eight horizontal pixels by six vertical pixels.
First, PN-sequence random-number data PN is defined as follows:
                    PN        =                  (                                                                      +                  1                                                                              -                  1                                                                              +                  1                                                                              +                  1                                                                              -                  1                                                                              +                  1                                                                              -                  1                                                                              -                  1                                                                                                      +                  1                                                                              +                  1                                                                              -                  1                                                                              -                  1                                                                              -                  1                                                                              +                  1                                                                              -                  1                                                                              +                  1                                                                                                      -                  1                                                                              +                  1                                                                              +                  1                                                                              -                  1                                                                              +                  1                                                                              +                  1                                                                              -                  1                                                                              +                  1                                                                                                      +                  1                                                                              -                  1                                                                              -                  1                                                                              -                  1                                                                              +                  1                                                                              +                  1                                                                              -                  1                                                                              -                  1                                                                                                      -                  1                                                                              -                  1                                                                              +                  1                                                                              +                  1                                                                              +                  1                                                                              -                  1                                                                              -                  1                                                                              +                  1                                                                                                      +                  1                                                                              +                  1                                                                              -                  1                                                                              +                  1                                                                              -                  1                                                                              -                  1                                                                              +                  1                                                                              -                  1                                                              )                                    Eq        .                                  ⁢                  (          1          )                    
The random-number data PN is generated so that the sum is statistically zero. Then, embedded information DC is spread by the random-number data PN having characteristics indicated by the above equation. When the polarity of the embedded information DC is expressed by “1”, the pattern of the random-number data PN is used without change. Then, the digital watermark pattern WM is expressed as follows:
                    WM        =                  PN          =                      (                                                                                +                    1                                                                                        -                    1                                                                                        +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        +                    1                                                                                        -                    1                                                                                        -                    1                                                                                                                    +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        -                    1                                                                                        +                    1                                                                                                                    -                    1                                                                                        +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        +                    1                                                                                                                    +                    1                                                                                        -                    1                                                                                        -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        -                    1                                                                                                                    -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        -                    1                                                                                        +                    1                                                                                                                    +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        +                    1                                                                                        -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        -                    1                                                                        )                                              Eq        .                                  ⁢                  (          2          )                    
When the polarity of the embedded information DC is expressed by “0”, the inverse pattern of the random-number data PN is used. That is, the digital watermark pattern WM is expressed as follows:
                    WM        =                              -            PN                    =                      (                                                                                -                    1                                                                                        +                    1                                                                                        -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        -                    1                                                                                        +                    1                                                                                        +                    1                                                                                                                    -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        +                    1                                                                                        -                    1                                                                                                                    +                    1                                                                                        -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        -                    1                                                                                                                    -                    1                                                                                        +                    1                                                                                        +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        +                    1                                                                                                                    +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        +                    1                                                                                        -                    1                                                                                                                    -                    1                                                                                        -                    1                                                                                        +                    1                                                                                        -                    1                                                                                        +                    1                                                                                        +                    1                                                                                        -                    1                                                                                        +                    1                                                                        )                                              Eq        .                                  ⁢                  (          3          )                    
If the embedded information DC is formed of a plurality of information bits, the frame data of the luminance signal may be divided into appropriate small regions so that each of the information bits is made to correspond with each of the regions. Alternatively, for example, a plurality of different orthogonal digital watermark patterns may be used so that each of the information bits is made to correspond with each of the digital watermark patterns. A combination of these methods may also be used.
In an image signal such as a digital video signal, it is assumed that frame data DV1 indicating a luminance signal pixel value of certain frame data is expressed as follows:
                              DV          ⁢                                          ⁢          1                =                  (                                                    50                                            51                                            52                                            54                                            52                                            52                                            50                                            49                                                                    49                                            50                                            51                                            53                                            54                                            53                                            50                                            50                                                                    48                                            50                                            50                                            50                                            51                                            52                                            49                                            48                                                                    49                                            49                                            50                                            48                                            49                                            50                                            50                                            49                                                                    48                                            48                                            50                                            49                                            47                                            50                                            52                                            50                                                                    49                                            50                                            52                                            51                                            51                                            52                                            55                                            53                                              )                                    Eq        .                                  ⁢                  (          4          )                    Since image signal such as digital video signals have a characteristic that adjacent luminance signals have almost the same pixel values, the values of adjacent pixels are set herein to approximately equal values.
The digital watermark is embedded by adding the digital watermark pattern WM to the frame data DV1 of the luminance signal. When the polarity of the embedded information DC is expressed by “1”, the digital watermark pattern WM expressed by Eq. (2) is added to the luminance signal expressed by Eq. (4), and frame data DV2 of the digital-watermarked luminance signal is expressed as follows:
                              DV          ⁢                                          ⁢          2                =                                            DV              ⁢                                                          ⁢              1                        +            WM                    =                      (                                                            51                                                  50                                                  53                                                  55                                                  51                                                  53                                                  49                                                  48                                                                              50                                                  51                                                  50                                                  52                                                  53                                                  54                                                  49                                                  51                                                                              47                                                  51                                                  51                                                  49                                                  52                                                  53                                                  48                                                  49                                                                              50                                                  48                                                  49                                                  47                                                  50                                                  51                                                  49                                                  48                                                                              47                                                  47                                                  51                                                  50                                                  48                                                  49                                                  51                                                  51                                                                              50                                                  51                                                  51                                                  52                                                  50                                                  51                                                  56                                                  52                                                      )                                              Eq        .                                  ⁢                  (          5          )                    
In order to detect the embedded information DC from the frame data DV2 of the digital-watermarked luminance signal, the same PN-sequence random-number data PN used for embedding the information is used. The inner product P1 of the frame data DV1 of the original luminance signal and the random-number data PN is given by the following equation:P1=DV1·PN=1  Eq. (6)
With the statistical characteristics of the image signal, the inner product P1 has a value close to zero. On the other hand, the inner product P2 of the frame data DV2 of the digital-watermarked luminance signal and the random-number data PN is given by the following equation when the polarity of the embedded information DC is expressed by “1”:
                                                                        P                ⁢                                                                  ⁢                2                            =                              DV                ⁢                                                                  ⁢                                  2                  ·                  PN                                                                                                        =                                                (                                                            DV                      ⁢                                                                                          ⁢                      1                                        +                    WM                                    )                                ·                PN                                                                                        =                                                (                                                            DV                      ⁢                                                                                          ⁢                      1                                        +                    PN                                    )                                ·                PN                                                                                        =                                                P                  ⁢                                                                          ⁢                  1                                +                                  PN                  2                                                                                                        =                              1                +                48                                                                        Eq        .                                  ⁢                  (          7          )                    
When the polarity of the embedded information DC is expressed by “0”, the inner product P2 is given by the following equation:
                                                                        P                ⁢                                                                  ⁢                2                            =                              DV                ⁢                                                                  ⁢                                  2                  ·                  PN                                                                                                        =                                                (                                                            DV                      ⁢                                                                                          ⁢                      1                                        +                    WM                                    )                                ·                PN                                                                                        =                                                (                                                            DV                      ⁢                                                                                          ⁢                      1                                        -                    PN                                    )                                ·                PN                                                                                        =                                                P                  ⁢                                                                          ⁢                  1                                -                                  PN                  2                                                                                                        =                              1                -                48                                                                        Eq        .                                  ⁢                  (          8          )                    
In this case, the absolute value of the inner product P2 is close to the inner product PN2 of the random-number data PN by itself. When the inner product P1 of the frame data DV1 of the original luminance signal and the random-number data PN, and the inner product P2 of the frame data DV2 of the digital-watermarked luminance signal and the random-number data PN are calculated with respect to various images, the distribution of the inner products P1 and P2 can be expressed using a probability density function shown in FIG. 15. A certain nonnegative threshold TH is set so as to discriminate portions with and without digital watermarks and to determine the polarity as follows:
P2 ≦ −TH:watermarked portion (polarity: 0)|P2| < TH:unwatermarked portionP2 ≧ TH:watermarked portion (polarity: 1). . . Ex. (9)
Accordingly, the embedded information DC can be detected from the frame data DV2 of the digital-watermarked luminance signal.
In practice, important points of digital watermarking are the reliability of digital watermark detection and the influence of digital watermarks on the image quality. In order to correctly discriminate portions with and without digital watermarks, it is necessary to define the threshold TH so that, in FIG. 15, the probability density function can be accurately separated into the “watermarked” portions and the “unwatermarked” portion. Actually, however, troughs overlap in the probability density function, and it is difficult to select the threshold TH so as to correctly discriminate portions with and without digital watermarks. The probability that an unwatermarked portion will be considered as a “watermarked” portion is referred to as “false positive”. An extremely small false positive value is required for ensuring secure distribution of content. In order to increase the reliability of digital watermark detection, therefore, the digital watermark embedding strength should be increased using a nonnegative scalar quantity C. Frame data DV2 of a luminance signal into which the digital watermark is embedded using the scalar quantity C with a large digital watermark embedding strength is given by the following equation:DV2=DV1+CWM  Eq. (10)
The inner product P2 of the frame data DV2 of the digital-watermarked luminance signal and the random-number data PN should be much larger. More specifically, the frame data DV2 is given by the following equation:
                                                                        P                ⁢                                                                  ⁢                2                            =                              DV                ⁢                                                                  ⁢                                  2                  ·                  PN                                                                                                        =                                                (                                                            DV                      ⁢                                                                                          ⁢                      1                                        +                    CWM                                    )                                ·                PN                                                                                        =                                                (                                                            DV                      ⁢                                                                                          ⁢                      1                                        ±                    PN                                    )                                ·                PN                                                                                        =                                                P                  ⁢                                                                          ⁢                  1                                ±                                  CPN                  2                                                                                        Eq        .                                  ⁢                  (          11          )                    
However, in the case where the digital watermark embedding strength is increased in this way, the influence of the digital watermark on the image quality is not negligible. There is a tradeoff between the reliability of digital watermark detection and the influence of the digital watermark on the image quality.
Techniques which effectively use human visual characteristics to embed a digital watermark have been proposed in order to reduce the influence of digital watermarks on the image quality as much as possible while maintaining high reliability of digital watermark detection. In such techniques, in view of the human visual characteristics, digital watermark patterns are reallocated in an image, or digital watermark patterns are matched with the motion of an image, thus effectively reducing the influence of the digital watermark on the image quality without changing the overall embedding strength. The human eye is sensitive to changes in low-frequency regions such as flat portions, but is insensitive to changes in high-frequency regions such as edge portions. By making use of this characteristic, digital watermark patterns are reallocated to imperceptible edge portions from perceptible flat portions, thus reducing the influence of the digital watermark on the image quality while maintaining high reliability of digital watermark detection. The digital watermark patterns are stationary in a still image, and the digital watermark patterns move together with moving images, thus achieving embedding of the digital watermark in a manner imperceptible to the human eye.
Techniques of directly superimposing additional information on an original information signal of image data, audio data, and so on, such as digital watermarking, provide a strong resistance to tampering. Such techniques are therefore expected to be secure information adding approaches.
In one specific digital watermarking method, for example, information such as a content ID for identifying content to be delivered on a network, or a user ID for identifying a user to which the content is directed is added to the content using digital watermarking before delivering the digitally watermarked content.
However, there has been a problem that, when the same content is assigned with different IDs for different users, a coalition of users can detect the IDs from the differences between a plurality of pieces of content and can perform signal processing to delete the IDs from the content so as to make the identification information undetectable, resulting in unauthorized secondary distribution of the content with the user IDs removed therefrom. In this situation, if content containing no ID originally is concurrently distributed, it is very difficult to identify tampered content.
If there are users having such tampered content, it is also difficult to identify the tamperer only by analyzing the content. In a case where content to be protected is illegally distributed, illegal-content searching such as content distribution route tracing or source finding is almost impossible, thus making it difficult to prove the illegality. Therefore, it is difficult to take appropriate action against tampered content.